This invention relates to batteries and, in particular, to separators for alkaline cells.
Alkaline batteries are generally cylindrical in shape and include an annular cathode disposed between the outer casing of the battery or can, and the anode material which occupies a cylindrical volume having a longitudinal axis generally coincident with that of the battery and can. Located between the cathode and the anode material is a separator which electrically insulates the cathode from the anode material, but which absorbs electrolyte and allows water transport and ion transfer between the electrodes. Heretofore, the separators used in alkaline batteries have generally been limited to commercially available battery separator papers and cellophane films.
While conventional battery separator paper has proven satisfactory, it would be desirable to provide methods and materials which would allow the separator to be installed at a lower cost using a simplified process and apparatus. In particular, the equipment used to cut and place the paper separators into the batteries are relatively complicated and expensive. Additionally, preparing the equipment used to cut and place the paper separators into the batteries requires sampling of the paper used to form the separators and adjustments of the equipment depending on the particular properties of the paper being used. Another problem with the use of paper separators is that process reliability is sensitive to the internal diameter of the cathode. For example, variations in the internal diameter of the cathode along the longitudinal length of the battery can result in areas wherein the separator does not intimately contact the cathode. Also, changes in anode basket volume affects cell performance. As a result, the interfacial area for ionic transport may be substantially reduced as compared with a battery having a separator basket with an internal diameter which does not vary along either the longitudinal or radial direction and wherein the separator is substantially in continuous contact along the entire internal cylindrical surface of the cathode. Another problem with paper separators is that because of the relatively complicated manipulations require to place the separators into the batteries, long process cycle times are required and process capability is generally low and varies widely between machines and even for any particular machine. A still further disadvantage with paper separators is that the paper takes up a substantial amount of volume within the cell, which, in turn, requires a substantial amount of electrolyte to wet the separator. Paper separators work optimally when wet and less efficiently when only damp. Also, the paper does not intimately contact the cathode over the entire cathode/separator interface, especially at the bottom of the separator where the folds occur, creating unused volume within the cell. Side seams of conventional paper separators also consume cell volume. A still further disadvantage with conventional paper separators is that the defect rate is greater than desired.
An alternative method for preparing an electrochemical cell which does not involve the use of a paper separator involves forming a polystyrene separator by placing a predetermined amount of polystyrene solution directly on the surface of a cathode and removing the organic solvent, thereby leaving a substantially continuous coating on the surface of the cathode. This method is generally undesirable and impractical because it typically requires placement of a reinforcing means on the surface of the cathode before application of the solution containing polystyrene, and requires evaporating large quantities of volatile organic solvents such as methylene chloride, tetrahydrofuran, ethyl acetate, acetone, benzene, toluene, and trichloroethylene. Placing of a reinforcing means on the surface of the cathode adds to the cost of the battery and requires complicated automation comparable to that required for automatically incorporating a paper separator into a battery. The use of volatile organic solvents is extremely undesirable due to health related issues (e.g., toxicity), safety related issues (e.g., flammability) as well as the difficulty and expense involved in avoiding environmental contamination. Some solvents if not entirely removed can detrimentally affect cell performance.
The invention has as an objective the provision of an improved method of constructing a battery using a separator which is installed within the battery without folding operations, and elimination of the various problems enumerated above relating to the use of conventional paper separators and polystyrene separators. A further objective of this invention is to provide batteries having improved volumetric efficiency, improved solvent transport characteristics across the separator, and improved ionic conductivity across the separator.
The above objectives are met, and the disadvantages with paper separators enumerated above are overcome by a battery having a separator formed directly on an electrode by applying a coating composition comprising a polymer or gel dispersed in a polar solvent to the surface of the electrode and solidifying materials in the applied coating composition.